IHIN OF 1 H I 111; KN HALS. 51 



have been also useful as an air-tight band around the animal, fastening the 

 mantle closely to the ahell. The very Blight impression made in tin- inner 

 surface of the living chamber shows thai this muscle was no! verv Btrong, 

 nor verv useful for purposes of prehension, and w< • with 



Dr. Waagen's remark, and oven perhaps ^>< a little further. Finely preei 



: . .mi ! •< t~ of Ammonoids, from Solenhofen and other places, 



of this annular band, and it seems t>> have been of :i similar 



nature, but of n ' importance in this order, as the pallia! muscles among 



mellibranchs. The animal of Nautilus was probably held in its shell almost 



- vely by pressure, and this band of muscles perhaps served t" Becure the 



: being disturbed by the mo. ij the outer pa 



the body while the animal was using its byponome. The supposed muscular 



band o figured b) Waagen, 1 runs forward on the Bides much 



the aperture than in Nautilus. This fact also 



the Ammonitinsa could not have used the fore pan- of the body 

 in the same way a< the Nautiloids, 



The i'"ii' the central rone of the Beptum is certainly a differentia] 



among Ammonoids when compared with Nautiloids. hut it is in strict correla- 

 tion with the arising and lengthening of the dorsal, ventral, and lateral lobes, 



illy tin- first two. and is therefore concomitant with the increasing com- 



>n <>t' the sutures the closer coiling, and the greater involution of the 

 whorl in this order. We have already L r i\eii the details sustaining this view 

 in our Gem I ephalopods, and need only refer here to the ca 



Pinna 11), in which the septa are double concaves on account of the 



formed by the large lateral Baddies, and the family of the Primordia- 

 rtill in the broad-whorled anarcestian Btage, the septa 



utiloideau i>r concave, but when the deep ventral" and dorsal* " lobes and 

 I Idles are formed, the septa become ammonitoid or convex along 



the median \-n>\" It becomes, therefore, necessary to look upon this differentia] 



ibu table directly to the habits Of the animal, and due to the 



imonoid to respond to the requirements of its surroundings. 



The pen itral position of the Biphon is a constant differential among 



\ the nosto - of the dura and Cretaceous, which 



:.1\ to physical changes, becoming uncoiled and departing in 



many of their - from the normal ancestral types, -till retained the 



of the siphon. Although there was considerable lateral \ 

 tion in the position of this organ in some species, it remained, bo far as known. 

 alwa I or ventral, 



The -1/.- .,t" the siphon of considerable importance in this 



and mu-t be considered as throwing -one- light on th 



point in tl I • balopoda. The siphon was n far less important 



ng the later than among the • laller, 



ii alread) gathered from what we have said above, in the adult 



IT 



